解：在?$Rt\triangle ABD$?中，?$∠B = 90°$?
根據(jù)勾股定理?$AB^2=AD^2-BD^2。$?
∵?$AD = 4，$??$BD = 1$?
∴?$AB^2=4^2-1^2= 15$?
又∵?$BC=BD + DC=1 + 3 = 4$?
?$ $?在?$Rt\triangle ABC$?中，?$∠B = 90°$?
根據(jù)勾股定理?$AC^2=AB^2+BC^2$?
∵?$AB^2=15，$??$BC = 4$?
∴?$AC=\sqrt {15+4^2}=\sqrt {15 + 16}=\sqrt {31}$?
